The present invention relates to a PLL (Phase-Locked Loop) circuit which synchronizes with a carrier of an AM signal when the AM signal is received, and with a pilot signal of a stereo signal when an FM signal is received.
FIGS. 1 and 2 respectively illustrate schematic block diagrams of conventional PLL circuits receiving AM and FM signals. Both the AM and the FM circuits have a phase detector 1, a VCO (Voltage-Controlled Oscillator) 2 and a phase shifter 3 arranged in a phase-locked loop (PLL). In the conventional PLL circuit because of the differing frequencies, it is necessary to provide two separate synchronizing circuits shown in FIGS. 1 and 2 for receiving the AM signal and for receiving the FM signal, respectively.
In the PLL circuit shown in FIG. 1 for the AM signal, an intermediate frequency AM signal as a reference signal is applied to the phase detector 1. An output signal of the phase detector 1 is applied to the phase shifter 3 through the VCO 2. An output signal of the phase shifter 3 is fed back to the phase detector 1, is subjected to phase comparison with the input AM signal, and then controls the VCO 2. The phase shifter 3 also provides a synchronizing signal as an output. In such a circuit, the phase shifter 3 shifts the phase of the output signal from the VCO 2 by .pi./2 so as to obtain the synchronizing signal from the PLL circuit, since the output signal of the VCO 2 has a phase difference of .pi./2 with respect to the reference signal (AM input signal) when the phase detector 1 is an analog multiplier type. When the PLL circuit shown in FIG. 2 receives the FM signal, an FM composite signal is applied to the phase detector 1. A pilot synchronizing signal is generated through the VCO 2 and the phase shifter 3.
In the above-described conventional device provided in an AM/FM receiver, it is necessary to provide two separate PLL circuits for receiving the AM signal and for receiving the FM signal. Thus, it causes the circuit to be complicated due to the large amount of the elements, and further, the cost may be high.